Problem with routes

gistfile1.txt

routes.rb
----------
Debateable::Application.routes.draw do

  resources :users do
    resources :posts
  end

  resources :topics do
    resources :posts
  end
end
----------

bundle exec rake routes:

user_propositions GET   /users/:user_id/posts(.:format)       posts#index
                  POST  /users/:user_id/posts(.:format)       posts#create

-----------
The first line of my form in views/posts/new.html.erb:
<%= form_for (@post) do |f| %>